Current computers are increasingly limited by power consumption and heat dissipation issues, roughly clamping the clock rate of the Central Processing Unit (CPU) at ˜3-4 GHz since around 2004. Most of this power (50-80%) is consumed in interconnects, i.e. the metal traces that move information around and on/off the chip. As these wires have become thinner and data-rates faster the overall energy efficiency has plummeted. This is one of the biggest challenges facing the computer industry today.
Optical interconnects are seen as the solution to this problem. Electrical data generated from electronic circuitry is encoded into a beam of light using an electro-optic modulator, transmitted via an optical cable/waveguide and converted back into electrical data using photo-detectors at the receiving end. Unlike an electrical wire, the limit on data transmission in an optical waveguide can be as high as 100 Tbit/s and data transfer at high bit rates is much more energy efficient.
In silicon based optical interconnects, there are primarily two modulation techniques employed: (a) interference based modulation, typically using Mach-Zehnder Interferometer (MZI) type modulators, and (b) resonance based modulation, typically using ring resonators. FIG. 1 shows a top view and a cross sectional view of (a) a MZI based modulator and (b) a ring resonator based modulator. In both cases, a thick oxide cladding (˜2 μm) is required to guide light in top silicon layer
In MZI modulators, optical modulation is achieved by changing the refractive index of one of the arms. This creates a phase difference between the two arms. By modulating this phase difference constructive and destructive interference can be achieved. This gives rise to intensity modulation at the output of the modulator. In ring resonator based modulators, the basic principle is to tune the ring in and out of resonance by changing its refractive index. In both these approaches, a material incompatibility arises when combining electronics and optics on the same silicon platform. This is because conventional optical waveguiding techniques require a lower cladding generally in the form of a thick buried oxide layer to guide light (as shown in FIG. 1). When integrated with electrical circuits such a layer traps heat in the electrical components, for example the transistors, and so reduces the integration density. This is unacceptable to the electronics industry. As a result, true integration of optical components with CMOS circuitry is not possible with conventional optical interconnect techniques.